The rotational speed of a transmission output element, such as a shaft or gear wheel, provides a useful indication of vehicle speed. Historically, rotational speed has been indicated mechanically on a revolution counter connected to a transmission output element by a rotating cable within a flexible tubular casing.
More recently speed indication has been provided by electrical signal from a suitable sensor, such as an inductive sensor; this arrangement avoids the need to provide a route path for a mechanical drive, and moreover the electronic speed signal may be useful in other vehicle systems such as anti-lock braking systems, traction control systems and the like. Speed sensing may be provided electronically from a vehicle wheel.
A typical electronic speed sensor relies upon rotational movement of a toothed wheel past a relatively stationary sensor. Several kinds of such sensor are known, and typically rely upon a non-contact inductive effect; the sensor may for example be a Hall-effect sensor. Although a speed sensor may respond to the passage of the teeth of a rotating gear wheel, more usually a toothed target wheel or tone wheel is provided on the rotating element whose speed is to be sensed.
When applied to a vehicle transmission, an electronic speed sensor is typically provided as an externally threaded component for placement in a threaded through hole of the transmission casing. The through hole must be in a location which can be reached for replacement and repair, and which is also convenient for a tone wheel. The compact nature of modern vehicles, and the variety of models in which a transmission may be used, makes the selection of a suitable location somewhat problematic.